On CNBC's "Mad Money Lightning Round", Jim Cramer said Allergan plc (NYSE: AGN) is an inexpensive stock and he would buy it. He explained that the stock is struggling at the moment, because the company has a drug that might come off patent.

Cramer would wait for USG Corporation (NYSE: USG) to come down. He would buy it after it pulls back.

Arena Pharmaceuticals, Inc. (NASDAQ: ARNA) has reinvented itself and now the stock is undervalued, thinks Cramer. He believes the stock is good.